Hypoxia in Coastal Marine Ecosystems

Annette Brickley (NES-LTER Education & Outreach Coordinator), Kathy Browne (Rider University), Gabi Smalley (Rider University)


Summary

Aquatic ecosystems are home to a complex intersection of physical and biological factors and an intersection of natural and anthropogenic factors. In the Chesapeake Bay, low oxygen events have occurred periodically and may be connected with harmful algal blooms, fish kills, heavy flooding/runoff events, and warming temperatures. Careful monitoring of the system by the Chesapeake Bay Program since 1984 allows scientists and policymakers to evaluate the causes of the events and monitor improvements in the health of the ecosystem.

Strengths of Module

Students will use a multi-variable 10-year data set from the Chesapeake Bay to investigate the change in dissolved oxygen concentration over time. To determine the possible cause(s) of hypoxic events, students will analyze additional variables and will need to relate and apply their understanding of natural and anthropogenic processes affecting the ecosystem. Students will support their conclusions with evidence from their analyses.

What does success look like?

Students will be able to:

  • Support a conclusion using evidence and their understanding of relevant scientific concepts.
  • Explain how environmental factors in aquatic ecosystems can cause hypoxic or anoxic events.
  • Make connections between natural and anthropogenic processes that affect aquatic ecosystems.
  • Use different data analysis techniques to quantify and/or validate the relationship between two or more variables.

Students will use appropriate and sufficient data sets and analyses of those data to support a conclusion about the cause of one or more hypoxic events. They will show their understanding of relevant scientific concepts in their reasoning, and their skills analyzing and interpreting data as evidence to support their conclusion(s).

Context for Use

This module was originally designed for use in an introductory oceanography course at Rider University, a private institution of 4000 students, with lecture size 25-30 students and lab size 12-16 students. The module is designed to take 3 hours if students have some background using spreadsheets and spreadsheet charts. Instructions are specific for use of Excel, but could be completed with other programs or graphing tools. Specific statistical analysis is not required, although some options are suggested if instructors and students are ready for such work. The module was designed to help students develop their own explanation for the hypoxic events by linking evidence with background knowledge about dead zones, fish kills, harmful algal blooms, etc.

While this activity was designed using Chesapeake Bay data, other aquatic ecosystems with time series data for oxygen, temperature, salinity, nutrients, and chlorophyll (and possibly pheophytin, particulate organic carbon (POC), or total suspended solids (TSS)) for both surface and bottom waters may be substituted. (Surprisingly, at the time of writing, the authors struggled to find other systems with all of these variables for the same location.)

For the Chesapeake Bay example included, data were downloaded from the Chesapeake Bay Program data hub at

https://www.chesapeakebay.net/what/downloads/cbp_water_quality_database_1984_present

Prior knowledge needed to complete Activities A & B:

  • Basics about photosynthesis, respiration, decomposition
  • Basic factors affecting plant growth (sunlight, water, nutrients)
  • Biological and physical processes affecting dissolved oxygen concentration: photosynthesis, respiration, decomposition, solubility of gases, turbulence/agitation
  • Properties of water that affect its density
  • Stability of stratified water
  • Some familiarity with Microsoft Excel will help

New learning will include:

  • Phytoplankton primary production
  • Decomposition in a 3-dimensional environment
  • Impact of nutrient limitation and excess on primary production and decomposition
  • Seasonal impacts on stratification and mixing
  • Using Excel to calculate and construct data visualizations (graphs)
  • Articulating trends and patterns in data
  • Articulating patterns across multiple data sets
  • Optional work: use of some appropriate statistical tests

Description and Teaching Materials

Why this Matters:

Students will develop their quantitative skills in the context of a real world issue to better understand the sensitivity of aquatic ecosystems [and fisheries] to human activity.

Quick outline/overview of the activities in this module

  • Activity A: Students learn to plot time series with 2 data sets (top and bottom measurements of dissolved oxygen); and understand that parameters at the surface can vary from bottom.
  • Activity B: Students do spreadsheet calculations and plot two different variables (dissolved oxygen and density) in a time series to begin to look at possible correlations.
  • Activity C: Students decide on how to determine the cause of hypoxic events by plotting additional biological variables and look for correlations and manageable measures.
  • Optional Activity D: Students extend their analyses of the data by completing cross correlation and Student's T-test calculations to quantify a lag time between variations in two variables in the data set provided.
  • Optional Activity E: Students develop an understanding of how common the issue is around the globe, and consider natural instances of higher productivity versus anthropogenically stimulated or enhanced instances.

Teaching Materials:

  • Student Handout (Microsoft Word 2007 (.docx) 1.7MB Jul19 21)
  • Dataset
    • Student Excel file (Excel 2007 (.xlsx) 999kB Jul19 21)

Teaching Notes and Tips

See Instructor's Manual.

Instructors should be mindful of misconceptions and student challenges such as:

  • Harmful algal blooms cause fish kills only because algae are toxic.
  • Water pollution is toxic chemicals or trash, not something like lawn or crop fertilizer.
  • Nutrients (nitrate, phosphate) are taken up by animals as "food."
  • Algae use up the oxygen in the water, causing low DO.
  • Temperature or salinity changes are directly responsible for lowering DO through the effect on gas solubility.
  • All water masses are the same.
  • Fertilizer pollution results from overuse only (vs. overuse AND/or minimally used land exposed to continued runoff when not used for crops) https://www.ecowatch.com/theres-nothing-average-about-this-years-gulf-of-mexico-dead-zone-1891172459.html
  • Be careful in the more open inquiry work for Activity C to not develop new misconceptions. See specifics in Activity C guide.
  • When data are studied, students are asked in numerous places in this lab to first describe patterns and only after doing so, explain the results.  We do this very intentionally to help students develop the skills to analyze data before coming to any conclusions about the data.  Often students have a tendency to jump to explanations but not include any evidence to support conclusions they make.

Workflow of this module:

  1. Assign any pre-class readings
  2. Give students their handout when they arrive to class
  3. Instructor gives brief slide presentation with background material. Discussion of the readings can be integrated into this presentation or done before.
  4. Students can then work through the module activities, breaking for discussion/presentation between each part, A, B, & C.

Notes on the student handout: 

Depending on how instructors would like students to record their process and answer questions, student handout may be reformatted.  Current format does not include space for student written responses.

Measures of Student Success

Part A: Students' graphs will demonstrate their abilities to make spreadsheet calculations, plot a time series, and make sense of the data.

Part B: Students' summaries of the patterns and trends they observed in their graphs will reveal their abilities to interpret and articulate their observations and key in on trends and patterns that might be most relevant for the phenomenon explored.

Part C: Students' explanations will show their understanding of relevant science concepts introduced in this module (or from previous learnings) and show how they support their conclusions with evidence and reasoning.

Optional D: Students will show, in their revised explanations, how statistical analysis can quantify observations and provide additional evidence to support conclusions.

Optional E: Students' explanations will show their ability to apply their background knowledge to consider global patterns in the location of "dead zones" and the potential impact of increasing sea surface temperature (due to climate change).

Rubric provided to assess explanations: Student Assessment Rubric- Hypoxia in Coastal Marine Ecosystems.docx (Microsoft Word 2007 (.docx) 19kB Jul19 21)

References and Resources

Instructor Resources:

Related EDDIE Modules

  • Harmful Algal Blooms (HABs) in Freshwater(coming soon)
  • Nutrient Monitoring in the Chesapeake Bay (coming soon)
  • Paleoclimate and Ocean Biogeochemistry